Historically, some aircraft engines have been started by devices external to the aircraft, which devices supply torque to accelerate the engine up to its self-sustaining speed within a required time. It has been long recognized that to minimize weight, cost and complexity, it is desirable to utilize equipment already on the aircraft for this purpose. The generating system can be used for engine starting by operating the generator as an electric motor using an external source of electrical energy. Typically, this source of energy is furnished by a ground service cart.
The arrangement just described is generally referred to as a generator-starter drive. The generator-starter drive requires a transmission capable of supplying constant speed to the generator in the normal generating mode as well as supplying torque in the reverse direction to the engine from the generator operating as a motor. The drive employed must allow the motor to come up to its constant operating speed under zero load, and then maintain constant speed while applying accelerating torque to the aircraft engine, which is initially at zero speed.
The Aleem U.S. Pat. No. 3,786,696 is typical of the advancing art involved in starter-drives and disclosures in FIG. 1, a starter-drive for use between an aircraft engine and a generator to transmit power in either direction. The Aleem arrangement includes a generator shaft 11, an engine shaft 10, a differential 14 for transmitting power from the engine shaft 10 to the generator shaft 11; a hydrostatic transmission 12, including one hydraulic unit 19 connected for rotation with the generator shaft; a second hydraulic unit 20 connected for rotation with a control gear 26 in the differential; and a first one-way clutch 16 connecting the second hydraulic unit 20 to drive the engine shaft 10 exclusively through the hydrostatic transmission 12 during starting. The Aleem starter-drive is therefore seen to require that all the torque from the generator/motor be delivered through the hydrostatic transmission. This requires that the size of the hydrostatic transmission be sufficient to handle the high starting torques necessary to overcome the inertia of the engine at rest or zero speed. The invention to be described hereinafter provides the same engine starting torques but employs a smaller and lower weight hydrostatic transmission and differential gear.
A more recent advancement in starter-drives is present in the Reynolds et al U.S. Pat. No. 3,274,855, which patent discloses in FIG. 1, a generator 11 drivingly connected through a differential 15' to a variable hydraulic unit 14, a normally fixed hydraulic unit 13 to an engine 10. In this arrangement during the starting mode, the generator 11 can be accelerated to synchronous speed by either short-circuiting the hydraulic lines between the hydraulic units 13, 14 or by varying the displacement of the variable hydraulic unit 14. Torque can then be supplied to the engine by throttling the short circuit in the former case, or by increasing displacement of the variable displacement unit in the latter case. The short-circuiting and throttling method allows both hydraulic units to be at full displacement and therefore, both units to apply maximum torque to the engine.
It is therefore apparent that Reynolds et al, not unlike Aleem, requires that all the torque from the generator/motor be delivered through the hydrostatic transmission 13, 14 with the attendant disadvantages previously noted. These disadvantages are overcome by the invention to be described more fully hereinafter.